Golf ball

ABSTRACT

The invention provides a golf ball having a core, a cover of at least one layer formed over the core, and a plurality of dimples on a surface of an outer layer of the cover. The ball has a coat of paint at the dimples, and non-dimple areas of the ball surface lack a coat of paint, leaving the outer cover layer exposed. The outer cover layer is formed of a material having a material hardness (Shore D) of less than 60. The dimples have a surface coverage (SR) of from 70 to 85%. 
     The golf ball of the invention has outstanding spin characteristics.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball which is endowed with an excellent spin performance, having a low spin rate on shots with a driver and a high spin rate on approach shots.

Most golf balls in use today are manufactured by employing, for example, injection molding or compression molding to encase a solid core that is generally composed primarily of a rubber such as diene rubber with a material composed primarily of urethane resin, ionomeric resin or the like.

The surface of the golf ball has numerous dimples formed thereon from the standpoint of aerodynamic properties, in addition to which it is painted to protect the appearance and enhance, for example, scuff resistance—such as from sand on bunker shots, weather resistance and stain resistance.

In recent years, there has been a rising demand not only for the above performance characteristics, but also for ball appearance, such as stylishness and visibility, which has become a major factor in evaluating golf balls. For example, JP-A 5-111550 and JP-B 03-32380 disclose methods for producing golf balls in which dimples and non-dimple areas are given differing colors so as to enhance the stylishness of the ball. The methods there disclosed for obtaining such golf ball involve coating the entire ball with paint of a different color than the color of the cover material, then removing the paint film in areas other than the dimples by surface grinding, and applying thereon a clear coating. The golf balls obtained by the above production methods can be imparted with various differing speckled appearances depending on the combination of the cover material color and the dimple color, and thus have excellent stylishness and visibility. However, because a clear protective layer has been formed on the surface, these golf balls are unable to manifest an adequate spin performance.

Hence, many users desire a golf ball which not only has an outstanding spin performance that provides both a good distance on shots with a driver and good controllability on approach shots, but also is highly aesthetic.

Other prior art relating to the present invention includes JP-A 60-027945 and JP-A 53-129460. However, these documents concern the aesthetic appearance of a golf ball and, at the very least, are unable to improve the spin performance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball that is endowed with an excellent spin performance, having a low spin rate on shots with a driver and a high spin rate on approach shots.

The inventors have conducted extensive investigations in order to attain the above object. As a result, they have found that, surprisingly, a finished golf ball (manufactured product) which has a coat of paint at the dimples but lacks a coat of paint at the land areas (non-dimple areas) is able, on shots with a driver in which the ball incurs a large deformation, to achieve a lower spin rate on account of the large influence of the dimples, and also is able, on approach shots in which the ball incurs a small deformation, to achieve a higher spin rate on account of the large influence of the land areas. Such a golf ball thus has an excellent spin performance that enables a spin rate optimal for the conditions of play to be achieved.

Accordingly, the invention provides the following golf balls.

-   [1] A golf ball comprising a core, a cover of at least one layer     formed over the core, and a plurality of dimples on a surface of an     outer layer of the cover, wherein the ball has a coat of paint at     the dimples and non-dimple areas (land areas) of the ball surface     lack a coat of paint, leaving the outer cover layer exposed, the     outer cover layer is formed of a material having a material hardness     (Shore D) of less than 60, and the dimples have a surface coverage     (SR) of from 70 to 85%. -   [2] The golf ball of [1], wherein the outer cover layer is composed     of polyurethane or polyurea. -   [3] The golf ball of [2], wherein the polyurethane or polyurea which     forms the outer cover layer is a thermoplastic polyurethane or a     thermoplastic polyurea. -   [4] The golf ball of [1], wherein the material which forms the outer     cover layer has a material hardness (Shore D) of 58 or less. -   [5] The golf ball of [1], wherein the land areas are places where,     after a coat of paint was formed over the entire ball, the outer     cover layer was then exposed by removing the paint coat. -   [6] The golf ball of [5], wherein the outer cover layer was exposed     by surface grinding to remove the paint coat at the land areas. -   [7] The golf ball of [1], wherein the outer cover layer is of a     color other than white.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing an example of the internal structure of the golf ball according to the present invention.

FIG. 2 presents schematic views of the production process for the golf ball of the present invention, illustrating the respective states of a dimple as the ball passes through (a) an outer cover layer forming step, (b) a painting step, and (c) a land area paint coat removing step.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention has a solid core (sometimes referred to below simply as the “core”), a cover of at least one layer, and a plurality of dimples formed on a surface of an outer layer of the cover. By arranging for the ball to have a coat of paint (paint film) only at the dimple areas, and to lack a coat of paint at non-dimple areas of the ball surface (land areas), the spin performance of the ball on shots with a driver and on shots with an iron is enhanced.

The construction of the inventive golf ball, so long as the ball includes a core and a cover of at least one layer and the ball has a coat of paint only at the dimple areas, may be suitably selected in a range that does not depart from the objects of the invention. For example, when the golf ball is a three-piece solid golf ball having a two-layer cover composed of an inner cover layer (intermediate layer) and an outer cover layer, as shown in FIG. 1, the ball will have a three-piece structure which includes at least a core 1, an inner cover layer (intermediate layer) 2 which encases the core 1, and an outer cover layer 3 which encases the inner cover layer 2. In cases where the ball has a multilayer structure with a cover of two or more layers, the layers are sometimes referred to collectively in this invention as the “cover.” That is, in the case of the three-piece solid golf ball shown in FIG. 1, the above-mentioned inner cover layer (intermediate layer) 2 and outer cover layer 3 are sometimes referred to collectively as the “cover.” A plurality of dimples 41 are formed on the surface of the outer cover layer 3. In addition, a paint coat 5 is formed at the dimples 41, and the dimples satisfy the subsequently described parameters of the invention. Areas of the ball surface other than the dimples 41 are land areas 42, and do not have a paint coat formed thereon. FIG. 1 shows a ball having a three-layer structure composed of a core 1, an inner cover layer (intermediate layer) 2 and an outer cover layer 3. As noted above, this construction may be suitably modified within a range that does not depart from the objects of the invention. For example, if necessary, the cover may be composed of only a single layer—namely outer cover layer 3, or it may be composed of three or more layers, including two or more inner cover layers (intermediate layer layers) 2. The core 1 may also be formed as a plurality of layers.

The method of obtaining the golf ball of the invention is not subject to any particular limitation, provided only the dimples 41 have a paint coat (paint film) 5 and the land areas 42 lack a paint coat (paint film). However, preferred use may be made of the method shown in FIG. 2. FIG. 2 presents schematic views illustrating an example of a process for obtaining the inventive golf ball. These diagrams show enlarged views of the vicinity of a dimple at successive steps in the process. That is, FIG. 2 shows the respective states of a dimple as the ball passes through (a) an outer cover layer forming step, (b) a painting step, and (c) a land area paint coat removing step. Although not shown here, the core 1 and the inner cover layer 2 are formed by conventional methods at stages preceding the above outer cover layer-forming step. The above steps (a) to (c) are each described in detail below.

First, the outer cover layer-forming step (a) is an operation in which an outer cover layer 3 is formed over the inner cover layer (intermediate layer). In this invention, the outer cover layer 3 may be formed by injection molding or the like using a known material, and is not subject to any particular limitation. Also, at the same time as the outer cover layer 3 is formed, a plurality of dimples 41 are formed on the surface thereof. Parting line flash that arises during molding of the outer cover layer 3 is removed by trimming.

The painting step (b) is an operation in which the ball G obtained in the above outer cover layer-forming step is painted. This step forms a paint coat (paint film) 5′ over the entire surface of the ball G. The paint used in this step is not subject to any particular limitation; use may be made of a known paint. From the standpoint of, e.g., hardness, stability, adhesive properties and heat resistance, a polyurethane resin paint, an epoxy resin paint or the like may be used. The use of a polyurethane resin paint is especially preferred. The painting method is also not subject to any particular limitation; use may be used of a known method, with the use of spray painting or the like being preferred. The ball G that has been painted in this step is dried under specific conditions, then furnished to the next operation. The ball prior to painting may be subjected to pretreatment such as plasma treatment in order to enhance adhesion between the outer cover layer 3 (ball surface) and the paint coat 5.

The paint coat-removing step is an operation in which, on the ball G that has been painted on the surface in the above painting step and has a paint coat 5′ over the entire surface thereof, the paint coat 5′ is removed from the land areas 42. Removal of the paint coat 5′ may be easily carried out by surface grinding. In this invention, suitable use may be made of a known grinding machine such as a spherical grinder. Removal of the paint coat 5′ at land areas 42 in this step enables a golf ball G having a paint coat 5 only at the dimples 41 to be obtained. Moreover, in the present invention, it is unnecessary that a clear coat or the like such as is used in the prior art is applied following this step.

The thickness of the paint coat 5 is set as appropriate for the ball specifications, and is not subject to any particular limitation, although the thickness is typically set to at least 0.012 mm, and preferably at least 0.014 mm. It is recommended that the thickness upper limit, although not subject to any particular limitation, be set to not more than 0.040 mm, and preferably not more than 0.038 mm. If the thickness of the paint coat 5 is too small, the durability of the paint coat 5 may decrease, possibly giving rise to peeling or the like. On the other hand, if it is too thick, the dimples 41 will be shallower, which may lower the distance traveled by the ball.

The golf ball of the invention can be easily obtained by the above-described method. At this time, the color of the ball as a whole, although not subject to any particular limitation, may generally be made white—the color used in most golf balls. In this case, the outer cover layer 3 may be made white, and the paint coat 5 may be made white or clear. If there is a concern that staining from dirt or the like and discoloration by ultraviolet light may be conspicuous at the land areas 42 where there is no paint coat, the outer cover layer 3 may be given a color other than white. In addition, the color of the outer cover layer 3 and the color of the paint coat 5 do not necessarily have to be the same; by making the respective colors different, it is possible to further enhance the aesthetics and visibility of the ball.

In this invention, because only the dimples have a paint coat 5 thereon whereas non-dimple areas of the ball surface (land areas 42) lack a paint coat, leaving the outer cover layer 3 exposed, although the reason why is not entirely clear, a difference appears to arise between shots with a driver, in which the club comes into contact with the paint coat 5 at the dimples 41 owing to large deformation of the ball, and approach shots, in which the club does not come into contact with the paint coat 5 at the dimples owing to small deformation of the ball. It is presumably on account of this that a lower spin rate can be achieved on shots with a driver, and a higher spin rate can be achieved on approach shots.

The golf ball of the invention has a core and a cover of at least one layer, in addition to which it has a paint coat only at the dimple areas. However, the materials of which the core and the cover are formed and the structures of these respective members are not subject to any particular limitation. The core and the cover are explained below in detail while referring to FIG. 1.

First, the core 1 may be formed using a rubber composition containing, for example, a known base rubber and also such ingredients as a co-crosslinking agent, an organic peroxide, an inert filler, sulfur and an organosulfur compound. In this case, it is preferable to use polybutadiene as the base rubber.

A co-crosslinking agent such as an unsaturated carboxylic acid or a metal salt thereof, an inorganic filler such as zinc oxide, barium sulfate or calcium carbonate, an organosulfur compound such as the zinc salt of pentachlorothiophenol, and an organic peroxide such as dicumyl peroxide or 1,1-bis(t-butylperoxy)cyclohexane may be suitably blended into the above base rubber. If necessary, commercial antioxidants and other ingredients may also be suitably added.

The diameter of the core 1, although not subject to any particular limitation, may be set to from 25 to 42 mm. In this case, the lower limit value is preferably at least 28 mm, more preferably at least 31 mm, and even more preferably at least 34 mm. The upper limit value may be set to preferably 41 mm or less, more preferably 40 mm or less, and even more preferably 39 mm or less.

Next, with regard to the cover, the number of cover layers and the material hardnesses (Shore D) and thicknesses thereof may be suitably set within ranges that do not depart from the objects of the invention, and are not subject to any particular limitations. For example, when the three-piece solid golf ball shown in FIG. 1 is manufactured by forming a two-layer cover composed of an inner cover layer (intermediate layer) 2 and an outer cover layer 3 over the above-described core 1, the material hardnesses and thicknesses of the respective cover layers may be set as described below. Here, “material hardness (Shore D)” refers to the hardness measured using a type D durometer in accordance with ASTM D2240 for a sheet obtained under the specific subsequently described conditions.

First, the thickness of the inner cover layer 2, although not subject to any particular limitation, may be set to at least 0.8 mm, preferably at least 1.2 mm, and more preferably at least 1.6 mm. It is recommended that the upper limit be 2.5 mm or less, preferably 2.2 mm or less, and more preferably 1.9 mm or less.

The material hardness (Shore D) of the outer cover layer 3 is set to less than 60, and may be set to preferably 58 or less, more preferably 56 or less, and even more preferably 54 or less. The lower limit value is not subject to any particular limit, although it may be set to at least 30, preferably at least 35, and more preferably at least 40. If the material hardness (Shore D) of the outer cover layer 3 is too high, the spin rate-increasing effect on approach shots may decrease. On the other hand, if it is too low, the moldability and surface grindability decrease, which may lead to a decline in manufacturability.

The outer cover layer 3 has a thickness which, although not subject to any particular limitation, may be set to at least 0.3 mm, preferably at least 0.5 mm, and more preferably at least 0.7 mm. It is recommended that the upper limit be 2.5 mm or less, preferably 2.2 mm or less, and more preferably 1.9 mm or less.

In the golf ball of the invention, the cover encasing the core may be formed of a known material. For example, use may be made of thermoplastic resins such as ionomeric resins, and various types of thermoplastic elastomers. Illustrative examples of these thermoplastic elastomers include polyester-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

In this invention, it is especially preferable for at least one cover layer to be formed of a resin composition which includes polyurethane or polyurea. In this case, the proportion of the overall resin composition accounted for by the polyurethane or polyurea, although not subject to any particular limitation, may be set to at least 50 wt %, and preferably at least 80 wt %. The polyurethane or polyurea used may be either one that is thermosetting or one that is thermoplastic. The polyurethane and polyurea used in the invention are described below.

Polyurethane

The structure of the polyurethanes which may be used in the invention includes an isocyanate, a long-chain polyol and a chain extender. The long-chain polyol employed here may be any that has hitherto been used in the art relating to thermoplastic polyurethanes, and is not subject to any particular limitation. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof.

No particular limitation is imposed on the method of producing the polyurethanes. Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and isocyanate are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

In cases where a polyurethane is employed as the cover material, use may be made of either the polyurethane material (I) or the polyurethane material (II) shown below. These materials are described in detail below.

Polyurethane Material (I)

This material (I) is composed primarily of components (A) and (B) below:

-   (A) a thermoplastic polyurethane material, -   (B) an isocyanate mixture obtained by dispersing (b-1) an isocyanate     compound having as functional groups at least two isocyanate groups     per molecule in (b-2) a thermoplastic resin that is substantially     non-reactive with isocyanate.

When the cover is formed with this polyurethane material (I), a golf ball having a better feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact can be obtained.

Next, each of above components is described.

The thermoplastic polyurethane material (A) has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol), and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol used as a starting material may be any that has hitherto been used in the art relating to thermoplastic polyurethane materials, but is not subject to any particular limitation. In the present invention, preferred use may be made of polyester polyols and polyether polyols. Illustrative examples of polyester polyols include adipate polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol, and lactone polyols such as polycaprolactone polyol. Illustrative examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol) and poly(tetramethylene glycol).

The chain extender employed is preferably one that has hitherto been used in the art relating to thermoplastic polyurethane materials, but is not subject to any particular limitation. In the present invention, use may be made of low-molecular-weight compounds with a molecular weight of 2,000 or less and having on the molecule at least two active hydrogen atoms capable of reacting with isocyanate groups. Of these, the use of aliphatic diols having from 2 to 12 carbons is preferred. Illustrative examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred.

The polyisocyanate compound is not subject to any particular limitation; preferred use may be made of one that has hitherto been used in the art relating to thermoplastic polyurethanes. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

A commercial product may be suitably used as the thermoplastic polyurethane material composed of the above-described material. Illustrative examples include those manufactured by DIC Bayer Polymer, Ltd. under the trade name Pandex, and those manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd. under the trade name Resamine.

The isocyanate mixture (B) is obtained by dispersing (b-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (b-2) a thermoplastic resin that is substantially non-reactive with isocyanate. Here, the isocyanate compound (b-1) is preferably one that is known to the art. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (b-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins, and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (b-2) and the isocyanate compound (b-1), expressed as the weight ratio (b-2):(b-1), to be within a range of from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (b-2) will exert a large influence, rendering inadequate the physical properties of the material. On the other hand, if the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too large, the isocyanate compound (b-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.

The isocyanate mixture (B) can be obtained by, for example, adding the isocyanate compound (b-1) to the thermoplastic resin (b-2) and thoroughly working together these components at a temperature of from 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as Crossnate EM30 (available from Dainichiseika Color & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

The above polyurethane material (I) is composed primarily of the above-described thermoplastic polyurethane material (A) and isocyanate mixture (B). In this polyurethane material (I), the isocyanate mixture (B) is included in an amount, per 100 parts by weight of the thermoplastic polyurethane material (A), of at least 1 part by weight, preferably at least 5 parts by weight, and more preferably at least 10 parts by weight. The upper limit in the amount included may be set to 100 parts by weight or less, preferably 50 parts by weight or less, and more preferably 30 parts by weight or less. If too little isocyanate mixture (B) is included relative to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, the unreacted isocyanate will discolor the molded material, which is undesirable.

In addition to the above-described components (A) and (B), another component (C), although not essential, may also be included in the polyurethane material (I). For example, thermoplastic polymeric materials other than thermoplastic polyurethane materials may be included; illustrative examples include polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, polyethylene and nylon resins. When such a component (C) is included, the amount included is selected as appropriate for such purposes as adjusting the hardness, improving the resilience, improving the flow properties, and improving the adhesion of the cover material. Although not subject to any particular limitation, this amount may be set to preferably at least 10 parts by weight per 100 parts by weight of component (A). The upper limit in the amount included, although not subject to any particular limitation, may be set to 100 parts by weight or less, preferably 75 parts by weight or less, and more preferably 50 parts by weight or less, per 100 parts by weight of component (A). Where necessary, various additives may be included in the polyurethane material (I). For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included.

A known molding method may be employed to mold the cover using the above polyurethane material (I). For example, use may be made of a method that involves adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, then feeding the resulting mixture to an injection molding machine and injecting the molten resin composition around the core. The molding temperature will vary according to the type of thermoplastic polyurethane material (A), but is generally in a range of from 150 to 250° C.

Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining in the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. In this case, the crosslinking reaction has not yet proceeded to a sufficient degree immediately after injection molding of the above polyurethane material (I), but the crosslinking reaction can be made to proceed further by carrying out an annealing step after molding, in this way conferring the golf ball cover with useful characteristics. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a fixed length of time, or aging the cover for a fixed period at room temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primary components are (D) a thermoplastic polyurethane and (E) a polyisocyanate compound. By forming a cover composed primarily of such a polyurethane material (II), it is possible to achieve an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resin blend is not fed as a plurality of types of pellets, but rather is supplied to, for example, an injection molding machine as one type of pellet prepared by incorporating a plurality of ingredients into the individual pellets.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the cover resin material. Specifically, it is recommended that the combined weight of above components (D) and (E) account for at least 60%, and preferably at least 70%, of the total weight of the cover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplastic polyurethane structure includes soft segments made of a polymeric polyol(polymeric glycol) that is a long-chain polyol, and hard segments made of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that has hitherto been used in the art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof.

Illustrative examples of polyester polyols that may be used include adipate polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol, and lactone polyols such as polycaprolactone polyol.

Illustrative examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. The polyether polyol may be used singly or as a combination of two or more thereof.

It is preferable for these long-chain polyols to have a number-average molecular weight in a range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in a range of 1,700 to 4,000, and even more preferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl number measured in accordance with JIS K-1557 (the same applies below).

Any chain extender that has hitherto been employed in the art relating to thermoplastic polyurethanes may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 1,500 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred, as the chain extender.

Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethanes may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component (D) to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic isocyanate compound to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction may be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing the thermoplastic polyurethane serving as component (D). Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and polyisocyanate compound are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

A commercial product may be used as the thermoplastic polyurethane serving as component (D). Illustrative examples include products available under the trade names Pandex T8295, Pandex T8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component (E), it is essential that, in at least some portion thereof within the single resin blend, all the isocyanate groups on the molecule remain in an unreacted state. That is, polyisocyanate compound in which all the isocyanate groups on the molecule are in a completely free state should be present within the single resin blend, and such a polyisocyanate compound may be present together with a polyisocyanate compound in which some of the isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as the polyisocyanate compound. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, using 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferred for achieving a good balance between the influence on moldability by, for example, the rise in viscosity accompanying reaction with the thermoplastic polyurethane serving as component (D), and the properties of the resulting golf ball cover material.

In the cover of the inventive golf ball, although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethanes may be included as component (F) in addition to above components (D) and (E). Including this component (F) in the above resin blend enables the flow properties of the resin blend to be further improved and enables various properties required of golf ball cover materials, such as resilience and scuff resistance, to be enhanced.

This component (F), which is a thermoplastic elastomer other than the above thermoplastic polyurethanes, is exemplified by one or more thermoplastic elastomer selected from among polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. The use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred because the resilience and scuff resistance are enhanced, owing to reactions with isocyanate groups, while at the same time a good manufacturability is retained.

The relative proportions of above components (D), (E) and (F) are not subject to any particular limitation. However, to fully achieve the advantageous effects of the invention, it is preferable for the weight ratio among the respective components to be (D):(E):(F)=100:2 to 50:0 to 50, and more preferably (D):(E):(F)=100:2 to 30:8 to 50.

In this invention, a cover-forming resin blend is prepared by mixing together component (D), component (E), and also optional component (F). At this time, it is essential to select the mixing conditions such that, of the polyisocyanate compound, at least some polyisocyanate compound is present in which all the isocyanate groups on the molecule remain in an unreacted state. For example, treatment such as mixture in an inert gas (e.g., nitrogen) or in a vacuum state must be furnished. The resin blend is then injection-molded over a core which has been placed in a mold. In order to smoothly and easily handle the resin blend, it is preferable for the blend to be formed into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficient isocyanate groups in an unreacted state remain in these resin pellets; the unreacted isocyanate groups react with component (D) or component (F) to form a crosslinked material while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter.

In addition, various optional additives may also be included in this cover-forming resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, and parting agents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is not subject to any particular limitation. However, to increase the flow properties and manufacturability, the MFR is preferably at least 5 g/10 min, and more preferably at least 6 g/10 min. If the melt mass flow rate of the resin blend is too low, the flow properties will decrease, which may cause eccentricity during injection molding and may also lower the degree of freedom in the thickness of the cover that can be molded. The melt mass flow rate is a measured value obtained in accordance with JIS K-7210 (1999 edition).

The method of molding the cover using the above material may involve feeding the above-described resin blend to an injection-molding machine and injecting the molten resin blend over the core. Although the molding temperature in this case will vary depending on the type of thermoplastic polyurethane, the molding temperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable though not essential to carry out molding in a low-humidity environment such as by purging with an inert gas (e.g., nitrogen) or a low-moisture gas (e.g., low dew-point dry air), or vacuum treating, some or all places on the resin paths from the resin feed area to the mold interior. Illustrative, non-limiting, examples of the medium used for transporting the resin include low-moisture gases such as low dew-point dry air or nitrogen. By carrying out molding in such a low-humidity environment, reaction by the isocyanate groups is kept from proceeding before the resin has been charged into the mold interior. As a result, polyisocyanate in which the isocyanate groups are present in an unreacted state is included to some degree in the molded resin material, thus making it possible to reduce variable factors such as an unnecessary rise in viscosity and enabling the real crosslinking efficiency to be enhanced.

Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the resin blend (cover material) that may be used in this invention is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10° C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150° C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observed from about 150° C., but a weight drop can be observed from about 230 to 240° C.

After the resin blend has been molded as described above, the properties as a golf ball cover can be additionally improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. “Annealing,” as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

Polyurea

A polyurea is a resin composition composed primarily of urea linkages formed by reacting (i) an isocyanate with (ii) an amine-terminated compound. This resin composition is described in detail below.

(i) Isocyanate

The isocyanate employed is preferably one that has hitherto been used in the art relating to thermoplastic polyurethanes, but is not subject to any particular limitation. Use may be made of isocyanates similar to those described above in connection with the foregoing polyurethane materials.

(ii) Amine-Terminated Compound

An amine-terminated compound is a compound having an amino group at the end of the molecular chain. In the present invention, the long-chain polyamines and/or amine curing agents shown below may be used.

A long-chain polyamine is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of from 1,000 to 5,000. In the present invention, the number-average molecular weight is more preferably from 1,500 to 4,000, and even more preferably from 1,900 to 3,000. In this average molecular weight range, golf balls having even better rebound, manufacturability, etc. are obtained. Illustrative, non-limiting, examples of the above long-chain polyamines include amine-terminated hydrocarbons, amine-terminated polyethers, amine-terminated polyesters, amine-terminated polycarbonates, amine-terminated polycaprolactones and mixtures thereof. These long-chain polyamines may be used singly, or as combinations of two or more thereof.

An amine curing agent is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of less than 1,000. In the present invention, the number-average molecular weight is more preferably less than 800, and even more preferably less than 600. Such amine curing agents include, but are not limited to, ethylenediamine, hexamethylenediamine, 1-methyl-2,6-cyclohexyldiamine, tetrahydroxypropylene ethylenediamine, 2,2,4- and 2,4,4-trimethyl-1,6-hexanediamine, 4,4′-bis(sec-butylamino)dicyclohexylmethane, 1,4-bis(sec-butylamino)cyclohexane, 1,2-bis(sec-butylamino)cyclohexane, derivatives of 4,4′-bis(sec-butylamino)dicyclohexylmethane, 4,4′-dicyclohexylmethanediamine, 1,4-cyclohexane bis(methylamine), 1,3-cyclohexane bis(methylamine), diethylene glycol di(aminopropyl)ether, 2-methylpentamethylenediamine, diaminocyclohexane, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, propylenediamine, 1,3-diaminopropane, dimethylaminopropylamine, diethylaminopropylamine, dipropylenetriamine, imido-bis-propylamine, monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, isophoronediamine, 4,4′-methylenebis(2-chloroaniline), 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, 3,5-diethylthio-2,4-toluenediamine, 3,5-diethylthio-2,6-toluenediamine, 4,4′-bis(sec-butylamino)diphenylmethane and derivatives thereof, 1,4-bis(sec-butylamino)benzene, 1,2-bis(sec-butylamino)benzene, N,N′-dialkylaminodiphenylmethane, N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine, trimethylene glycol di-p-aminobenzoate, polytetramethylene oxide di-p-aminobenzoate, 4,4′-methylenebis(3-chloro-2,6-diethyleneaniline), 4,4′-methylenebis(2,6-diethylaniline), m-phenylenediamine, p-phenylenediamine and mixtures thereof. These amine curing agents may be used singly or as combinations of two or more thereof.

(iii) Polyol

Although not an essential component, in addition to the above-described components (i) and (ii), a polyol may also be included in the polyurea. In the present invention, this polyol is preferably one that has hitherto been used in the art relating to thermoplastic polyurethanes, but is not subject to any particular limitation. Specific examples include the long-chain polyols and/or polyol curing agents mentioned below.

The long-chain polyol may be any that has hitherto been used in the art relating to thermoplastic polyurethanes. Examples include, but are not limited to, polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof.

The long-chain polyol has a number-average molecular weight of preferably from 1,000 to 5,000, more preferably from 1,500 to 4,000, and even more preferably from 1,900 to 3,000. In this average molecular weight range, golf balls having even better rebound, manufacturability and the like are obtained.

The polyol curing agent is preferably one that has hitherto been used in the art relating to thermoplastic polyurethanes, but is not subject to any particular limitation. In the present invention, use may be made of a low-molecular-weight compound having on the molecule at least two active hydrogen atoms capable of reacting with isocyanate groups, and having a molecular weight of less than 1,000. Of these, the use of aliphatic diols having from 2 to 12 carbons is preferred. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, the use of 1,4-butylene glycol is especially preferred. The polyol curing agent has a number-average molecular weight of preferably less than 800, and more preferably less than 600.

Where necessary, various additives may also be included in the polyurea. For example, pigments, inorganic fillers, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may be suitably included.

A known method may be used to produce the polyurea. A known method such as a prepolymer process or a one-shot process may be suitably selected.

The method of molding the cover using the above polyurethane materials (I) and (II) and the above polyurea may involve, for example, feeding these materials to an injection-molding machine and injecting them around the core. Although the molding temperature in such a case will vary depending on the formulation and other factors, the molding temperature will generally be in a range of from 150 to 250° C.

In the present invention, dimples having the following parameters (1) to (3) are formed on the surface of the cover formed of the above-described materials. In cases where the surface of the ball is subjected to finishing treatment (e.g., painting, stamping, surface grinding) after the cover has been formed, parameters (1) to (3) below are calculated based on the shape of the dimples on the finished ball in which all such treatment has been completed.

Dimple Parameter (1)

The total number of dimples is not subject to any particular limitation, but may be set in a range of at least 250 but not more than 500. In this case, the lower limit value is set to preferably at least 280, more preferably at least 300, and even more preferably at least 310. The upper limit value is set to preferably 450 or less, more preferably 420 or less, and even more preferably 400 or less.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR), defined as the sum of the surface areas on a hypothetical sphere that are circumscribed by the edges of the respective dimples as a proportion of the surface area of the hypothetical sphere, is set to preferably at least 70%, more preferably at least 72%, and even more preferably at least 74%. The upper limit value of SR is set to preferably 85% or less, more preferably 83% or less, and even more preferably 81% or less. At an SR value lower than the above range, adequate aerodynamic properties may not be achieved, in addition to which the proportion of land areas (areas without a paint coat) increases. As a result, when discoloration of the cover occurs, the discoloration may be conspicuous, worsening the appearance of the ball. On the other hand, an SR value that is too high may lead to a decrease in the spin rate-lowering effects on shots with a driver.

Dimple Parameter (3)

The dimple volume ratio (VR), defined as the sum of the volumes of individual dimple spaces below the surface of a hypothetical sphere were the golf ball to have no dimples on the surface as a proportion of the volume of the hypothetical sphere, is not subject to any particular limitation. However, to improve the aerodynamic performance, VR may be set to from 0.6 to 1%.

The shapes of the dimples are not limited to circular shapes, and may also be suitably selected from among, for example, polygonal, tear-shaped and oval shapes. Setting the number of dimple types to at least three, and preferably at least five, makes it possible for the dimples to cover a higher proportion of the spherical surface. Also, by interspersing large and small dimples so as to increase the surface coverage to the specified range, extreme fluctuations in the coefficient of lift (CL) within the low-velocity region can be suppressed, enabling the ball trajectory to be made relatively low and thus making it easier to elicit the advantageous effects of the invention. In such cases, although not subject to any particular limitation, the dimple diameter (in polygonal dimples, the diagonal length) may be set to from 0.5 to 6 mm, and the dimple depth may be set to from 0.05 to 0.5 mm.

The golf ball of the invention can be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm and a weight of not more than 45.93 g. The upper limit value for the diameter may be set to preferably not more than 44.0 mm, more preferably not more than 43.5 mm, and even more preferably not more than 43.0 mm. The lower limit value for the weight may be set to preferably not less than 44.5 g, more preferably not less than 45.0 g, even more preferably not less than 45.1 g, and most preferably not less than 45.2 g.

As described above, in the golf ball of the present invention, a paint coat is formed only at the dimples formed on the surface of the outer cover layer; a paint coat is not formed on the surface of the outer cover layer in non-dimple areas (land areas). The golf ball in this state serves as the finished product (manufactured product); unlike in the prior art, the golf ball does not have a finish coat such as a clear coat applied thereto. Such a construction enables the golf ball of the invention to manifest a spin performance that is optimal for the conditions of play.

EXAMPLES

The following Examples and Comparative Examples are provided by way of illustration and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 to 3 1. Production of Solid Core

The rubber compositions shown in Table 1 were prepared, then molded and vulcanized at 155° C. for 20 minutes to produce solid cores having a diameter of 37.7 mm.

TABLE 1 Formulation Example Comparative Example (pbw) 1 2 3 1 2 3 Polybutadiene 100 100 100 100 100 100 Zinc acrylate 28 28 28 28 28 28 Zinc oxide 4 4 4 4 4 4 Barium sulfate 22 22 22 22 22 22 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 Zinc salt of 0.6 0.6 0.6 0.6 0.6 0.6 pentachlorothiophenol Peroxide 1.2 1.2 1.2 1.2 1.2 1.2

Details of the materials mentioned in Table 1 are as follows.

-   Polybutadiene rubber:     -   cis-1,4-Polybutadiene, available under the trade name “BR 730”         from JSR Corporation. -   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Zinc oxide: Grade 3 zinc oxide, available from Sakai Chemical     Industry Co., Ltd. -   Barium sulfate: Available under the trade name “Precipitated Barium     Sulfate #100” from Sakai Chemical Industry Co., Ltd. -   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi     Shinko Chemical Industry Co., Ltd. -   Zinc salt of pentachlorothiophenol:     -   Available from Zhejiang Cho & Fu Chemical Co., Ltd. -   Peroxide: 1,1-Bis(t-butylperoxy)cyclohexane, available under the     trade name “Perhexa C-40” from NOF Corporation.

2. Formation of Intermediate Layer and Outer Cover Layer

Next, the intermediate layer-forming material shown in Table 2 and the outer cover layer-forming material shown in Table 3 were successively injection-molded over the core manufactured as described above, thereby forming around the core an intermediate layer and an outer cover layer of the respective thicknesses shown in Table 4. The outer cover layer-forming material was prepared by using a twin-screw extruder to blend together the various starting materials shown in Table 3 (units: parts by weight) under a nitrogen gas atmosphere. The material thus obtained was in the form of pellets having a length of 3 mm and a diameter of 1 to 2 mm.

TABLE 2 Formulation Example Comparative Example (pbw) 1 2 3 1 2 3 Himilan 1605 69 69 69 69 69 69 Dynaron 6100P 31 31 31 31 31 31 Behenic acid 18 18 18 18 18 18 Polytail H 2 2 2 2 2 2 Calcium 2.3 2.3 2.3 2.3 2.3 2.3 hydroxide Calcium 0.15 0.15 0.15 0.15 0.15 0.15 stearate Zinc stearate 0.15 0.15 0.15 0.15 0.15 0.15 Phthalocyanine 0.008 0.008 0.008 0.008 0.008 0.008 green Phthalocyanine 0.002 0.002 0.002 0.002 0.002 0.002 blue

Details of the materials mentioned in Table 2 are as follows.

-   Himilan 1605: A sodium-neutralized ionomer available from     DuPont-Mitsui Polychemicals Co., Ltd. Acid content, 15 wt %.     Material hardness (Shore D), 65. -   Dynaron 6100P: An olefinic thermoplastic elastomer available from     JSR Corporation. -   Behenic acid: Available under the trade name “NAA-2225 (Powder)”     from NOF Corporation. -   Polytail H: A low-molecular-weight polyolefin polyol available from     Mitsubishi Chemical Corporation. -   Calcium hydroxide: Available under the trade name “CLS-B” from     Shiraishi Calcium Kaisha, Ltd. -   Zinc stearate: Available from NOF Corporation

TABLE 3 Formulation Example Comparative Example (pbw) 1 2 3 1 2 3 Pandex T8260 100 Pandex T8295 75 Pandex T8290 75 75 25 75 75 Pandex T8283 25 25 25 25 Polyisocyanate compound 8 8 8 8 8 8 Polyethylene wax 1 1 1 1 1 1 Titanium oxide 3 3 3 3 3 3

Details of the materials mentioned in Table 3 are as follows.

-   Pandex T8260: A MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer, Ltd. Resin hardness (Shore D), 60. -   Pandex T8295: A MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer, Ltd. Resin hardness (Shore A), 95. -   Pandex T8290: A MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer, Ltd. Resin hardness (Shore A), 90. -   Pandex T8283: A MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer, Ltd. Resin hardness (Shore A), 83. -   Polyethylene Wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd. -   Titanium Oxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd.

3. Painting Step

After spraying a clear polyurethane resin coating (available from Asia Industry Co., Ltd.) onto the surface of the three-piece solid golf ball obtained as described above, drying was carried out by heating at 55° C. for 45 minutes, thereby giving a three-piece solid golf ball having a 0.015 mm thick paint coat (paint film) on the ball surface. The ball at this point was treated as Comparative Example 1.

4. Paint Coat Removing Step (Surface Grinding Step)

The surface of the golf ball obtained as described above was ground using a spherical grinder to remove the paint coat at the land areas, thereby giving a three-piece solid golf ball having a paint coat only at the dimples.

The properties of the resulting golf ball were examined by the following methods. The results are shown in Table 5.

Diameters of Core and Intermediate Layer-Covered Sphere

The diameter was measured at five randomly selected places on the surface, measurement being carried out at a temperature of 23±1° C., and the average of these measurements was taken as the measured value for a single core or intermediate layer-covered sphere. The average value was then determined for five measured cores or intermediate layer-covered spheres.

Ball Diameter

The diameter was measured at 15 randomly selected dimple-free places, measurement being carried out at a temperature of 23±1° C., and the average of these measurements was taken as the measured value for a single ball. The average value was then determined for ten measured balls.

Golf Ball Deflection

The golf ball was compressed at a temperature of 23±1° C. and a speed of 10 mm/s, and the deflection (mm) from the state where the ball was subjected to an initial load of 98 N (10 kgf) until it was subjected to a final load of 1,275 N (130 kgf) was measured. The average value for 10 measured golf balls was determined.

Cover Material Hardness (Shore D Hardness)

The cover-forming resin material was melted at a suitable temperature in a range of 200 to 220° C. with an injection molding machine, molded into the form of a sheet having a thickness of 2 mm using an injection mold equipped with a 1 mm thick film gate under suitable conditions such that scorching and sink marks do not form, and cooled for 20 seconds at a cooling temperature of 20° C., following which the sheet was removed from the mold and stored for two weeks in a 23±1° C. environment. Three sheets of the resulting specimens were stacked to a thickness of 6 mm or more, following which the hardness was measured at 12 places with a type D durometer (in accordance with ASTM D2240-95) within a 23±2° C. environment. After excluding the largest and smallest measurements, the average for the remaining ten places was determined.

Spin Performance (Driver)

A driver W#1 was mounted on a golf swing robot, the initial backspin rate by the ball when hit at a head speed of 45 m/s was measured, and the average for ten shots was determined. The conditions that result in an initial ball velocity of about 65 m/s, a launch angle of about 10.5° and an initial backspin rate of about 2,500 rpm when a TourStage X-01B+ golf ball (manufactured by Bridgestone Sports Co., Ltd.) is hit at the above-indicated head speed were set as the striking conditions. The club used was a TourStage X-Drive 405 HR (loft, 10.5°), which is manufactured by Bridgestone Sports Co., Ltd. The following criteria were used for rating the spin performance.

Good: less than 2,550 rpm

NG: 2,550 rpm or more

Spin Performance (Wedge)

A sand wedge was mounted on a golf swing robot, the initial backspin rate by the ball when hit at a head speed of 15 m/s was measured, and the average for ten shots was determined. The conditions that result in an initial ball velocity of about 10 m/s, a launch angle of about 27° and an initial backspin rate of about 3,600 rpm when a TourStage X-01B+ golf ball (manufactured by Bridgestone Sports Co., Ltd.) is hit at the above-indicated head speed were set as the striking conditions. The club used was a TourStage TW-03 (loft, 57°) manufactured by Bridgestone Sports Co., Ltd. The following criteria were used for rating the spin performance.

Good: less than 3,500 rpm

NG: 3,500 rpm or more

TABLE 4 Example Comparative Example 1 2 3 1 2 3 Core diameter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 Core weight (g) 32.9 32.9 32.9 32.9 32.9 32.9 Intermediate layer outside 41.1 41.1 41.1 41.1 41.1 41.1 diameter (mm) Intermediate layer weight (g) 40.6 40.6 40.6 40.6 40.6 40.6 Intermediate layer thickness 1.7 1.7 1.7 1.7 1.7 1.7 (mm) Outer layer thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 Ball diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Ball weight (g) 45.5 45.7 45.6 45.6 45.5 45.7 Ball deflection (mm) 2.7 2.6 2.6 2.7 2.6 2.5 Cover material hardness 47 47 53 47 47 60 (Shore D) Dimple Number 344 378 344 344 326 344 parameters VR 1.50 1.30 1.50 1.50 1.60 1.50 SR 80.1 74.2 80.1 80.1 90.0 80.1 Surface grinding after painting yes yes yes no yes yes Spin Driver Spin rate 2,530 2,548 2,498 2,564 2,577 2,512 performance (rpm) Rating good good good NG NG good Wedge Spin rate 3,712 3,704 3,543 3,647 3,706 3,376 (rpm) Rating good good good good good NG 

1. A golf ball comprising a core, a cover of at least one layer formed over the core, and a plurality of dimples on a surface of an outer layer of the cover, wherein the ball has a coat of paint at the dimples and non-dimple areas (land areas) of the ball surface lack a coat of paint, leaving the outer cover layer exposed, the outer cover layer is formed of a material having a material hardness (Shore D) of less than 60, and the dimples have a surface coverage (SR) of from 70 to 85%.
 2. The golf ball of claim 1, wherein the outer cover layer is composed of polyurethane or polyurea.
 3. The golf ball of claim 2, wherein the polyurethane or polyurea which forms the outer cover layer is a thermoplastic polyurethane or a thermoplastic polyurea.
 4. The golf ball of claim 1, wherein the material which forms the outer cover layer has a material hardness (Shore D) of 58 or less.
 5. The golf ball of claim 1, wherein the land areas are places where, after a coat of paint was formed over the entire ball, the outer cover layer was then exposed by removing the paint coat.
 6. The golf ball of claim 5, wherein the outer cover layer was exposed by surface grinding to remove the paint coat at the land areas.
 7. The golf ball of claim 1, wherein the outer cover layer is of a color other than white. 